268 



WHALES 



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Figure 146. Two krill legs (Euphausia sviperba Dana) greatly magnified to show their 

 straining properties. {Barkley, ig4o.) 



little later, the mouth is closed for a brief interval, the tongue is brought up 

 and the krill pushed towards the throat in a way that is not yet fully vinder- 

 stood. Rorquals, whose mouths can be greatly widened by virtue of their 

 system of external folds and grooves, take in large quantities of krill with 

 one gulp, close the mouth, and contract the muscles of the tongue and the 

 base of the mouth, thus squeezing the water between the baleen and 

 expelling it over the edge of the lower jaw. Once the mouth has been 

 closed, the krill is pushed towards the throat. 



We have seen that very large sharks also feed on plankton and par- 

 ticularly on little shrimps. The reader might like to know, therefore, that 

 these animals strain their food in practically the same way as whales; 

 their branchial arches have a complete system of small ossified plates on 

 the inside. It is not known how giant reptiles obtained and strained their 

 aquatic prey, though some species may have used their teeth. On the other 

 hand, they may have had a softer structure analogous to baleen which, 

 being made of more delicate tissue than bone, failed to fossilize. The small 

 capacity of the mouth of these giant reptiles, however, does not support 

 the hypothesis that they were plankton feeders (F. C. Fraser). Krill itself 

 also has a kind of plankton strainer, although its 'mesh' is of course 

 infinitely finer than that formed by the baleen. Krill feeds mainly on 

 smooth diatoms less than o -04 mm. in size, and their favourite food is 



Figure 14/. Lower jaw of Basilosaurus cetoides Owen, a serpent-shaped Archaeocele, 

 showing dentition. I - incisors; C = canines; P = premolars; M -= molars. {Kellogg, 



1936.) 



